it's been a while since I've messed with IGMP, it's possible I never even tried on the default LAN as my usual practice is discarding the default vlan 1, defining vlan's for everything and putting ports in them as needed (then you never need dual-mode tagging for one, since you will never have to pass the "1" tag, which has to be untagged)
you can certainly define a lag without shutting down ports, but before you define the lag, if you plug both links, it has no way of knowing they're going to be a lag in the future, so it's just going to see a loop and shut them down if stp is enabled (or allow a broadcast storm if it's not). What behavior were you expecting? if RSTP is on, and you plug in two links which make a loop, it's going to get disabled regardless of vendor
you can certainly define a lag without shutting down ports, but before you define the lag, if you plug both links, it has no way of knowing they're going to be a lag in the future, so it's just going to see a loop and shut them down if stp is enabled (or allow a broadcast storm if it's not). What behavior were you expecting? if RSTP is on, and you plug in two links which make a loop, it's going to get disabled regardless of vendor